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Surf, Sand, and Stone
How Waves, Earthquakes, and Other Forces Shape the Southern California Coast
By Keith Heyer Meldahl
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2015 The Regents of the University of California
All rights reserved.
Time, Faults, and Moving Plates
A Recipe for Southern California
Time is Nature's way of keeping everything from happening at once.
— John Wheeler
In one second, the backyard of an oceanfront home may disappear as the bluff beneath it collapses. In one day, a storm may sweep away a beach. In one year, some Californians will see cracks open in their lawns and driveways as their property oozes downhill on slow-moving landslides. In any given decade, the odds are good that a big earthquake will shake California. In fifteen million years, the slow creep of the Earth's tectonic plates will put Los Angeles next to San Francisco. The processes that shape our world operate over a vast range of time scales, from seconds to millions of years. But our lives encompass events on the short end of Nature's clock, and that makes it hard to appreciate the power of long-term change. To understand how the Southern California coast came to be, we need to move beyond human time and get our minds around geologic time, or what geologists call deep time. The reason is simple. Geological processes such as erosion, the uplift of mountains, or the movements of the Earth's tectonic plates may seem trivially slow over human time. But give these processes millions of years, and they can accomplish stunning work.
We all know big numbers when we see them, but the years of deep time — millions and billions of years — are hard to grasp. Big numbers are always hard to grasp. Who among us, for example, has a good feel for one billion of anything? One billion dollars is close to Bill Gates's annual income, so here's a way to put such a number into perspective: one billion dollars per year ÷ 365 days per year ÷ 24 hours per day ÷ 60 minutes per hour ÷ 60 seconds per minute = $31.71 per second. In other words, Bill Gates earns about $32 per second, around the clock, all year long. Now imagine that Gates is walking down the sidewalk when someone stops him and says, "Bill, I need some financial advice. I'll pay you for whatever your time is worth." Gates responds, "OK, that'll be $96 for the last three seconds. How else can I help you?"
If that scenario gave you a better sense of one billion, let's try for 4.56 billion — the age of the Earth in years. Imagine a ninety-five-gallon bathtub (a large household bathtub) filled to the brim with medium-grained sand. (Geologists classify sand precisely, so "medium-grained" means that the grains range from 0.25 to 0.5 millimeters in diameter, which is typical of many beach sands, as well as standard table salt.) Let each grain represent one year. Wet one fingertip and dip it in the brim-full tub. About five hundred grains cling to your fingertip, representing roughly the number of years since Columbus crossed the Atlantic. Scoop up one-eighth of a teaspoon, or about eight thousand grains. That represents the years since the dawn of human agriculture. Now scoop up a heaping tablespoon, roughly two hundred thousand grains. You hold the entire existence of our species, Homo sapiens. The brim-full bathtub holds about 4.5 billion grains, representing the age of the Earth. Pour that tablespoon back. Do you see a difference? Compared to deep time, human time is virtually nonexistent.
What does the vastness of geologic time mean for the formation of California? You might think I'm about to make an argument for California's ancientness, but no. Geologically speaking, California is young — although still almost unfathomably old by human standards. Two hundred million years ago (about four gallon-buckets of sand in the tub), California didn't yet exist. North America ended in what is now western Nevada, and had you stood then, say, where Reno is today, you would have gazed west not at the Sierra Nevada and the rest of California, but at ocean waves and deep blue sea. Had you watched for the next hundred million years, you would have seen California arrive, piece by piece, from the ancient Pacific Ocean. Islands, seamounts, and vast chunks of ocean floor, carried by the Earth's tectonic plates, landed on the continent's edge, one behind the other, to assemble California. (I'll give you a fuller explanation of how this happened — and how we know — in chapter 4.) About twenty million years ago, mighty faults, some of them precursors of the modern San Andreas fault, began to slice up this collection of imported rock and send it on the move yet again. That interval — the past twenty million years — is my focus in this book. That's not much compared to the age of the Earth. In the bathtub analogy, it's about seven cups of sand. But it's still a vast span — enough deep time to pack a wallop.
Here's a story to show you how.
MEXICAN PEBBLES FAR FROM HOME
In the ocean thirty miles south of Santa Barbara lie the four Northern Channel Islands — San Miguel, Santa Rosa, Santa Cruz, and Anacapa from west to east — stretching west in a line out to sea from the end of the Santa Monica Mountains near Los Angeles. San Miguel faces six thousand miles of open Pacific Ocean, and thus gets blasted by some of the fiercest winds and largest waves anywhere in Southern California. The day I hiked across the island was typical, with a fierce wind yanking at my hat, shooing sand across the dunes, and tearing spray off the cresting swells. Winding my way down through the dunes to Simonton Cove, on the island's western shore, I found some very special pebbles in beach outcrops scrubbed clean by the waves. The pebbles were smooth and round, and on average about the size of a baseball. They lay encased in upended layers of Eocene sand and gravel. They were not, I knew, native to San Miguel Island — or even to California. These pebbles were emigrants from Mexico.
Rock made of many smooth, rounded pebbles is called conglomerate, and it forms wherever breaking waves or flowing rivers tumble rock pieces and wear them smooth. Conglomerate is a common rock, but the pebbles in the conglomerate on San Miguel Island contain an uncommon curiosity: distinctive purple-maroon pebbles of rhyolite — a type of volcanic rock — sparkling with crystals of quartz and feldspar (figure 1.1). The rhyolite pebbles are so distinct — both visually and in their detailed chemical makeup — that geologists can confidently trace their origin to the exact volcanoes from which they eroded. Incredibly, those volcanoes, now long dead, are in Sonora, Mexico — more than five hundred miles from San Miguel Island. And that's not all. In the San Diego suburb of Poway, and in sea cliffs by La Jolla, you can find pebbles that are dead ringers for the ones on San Miguel Island. These, too, could only have come from near the same volcanoes in Sonora. How did pebbles eroded from Mexican volcanoes migrate two hundred fifty miles to San Diego and five hundred miles to San Miguel Island (figure 1.2)?
The answer lies in the relentless creep of the Earth's tectonic plates. San Miguel Island today lies mostly on the Pacific Plate. The remains of the Mexican volcanoes lie on the North American Plate. San Diego occupies the fault-slivered zone in between (figure 1.3). The Pacific and North American Plates are sliding side-by-side past each other about two inches per year (a number first determined by matching up rock bodies split when the Gulf of California began to open about 5.5 million years ago, and confirmed today by global positioning system [GPS] measurements). That side-by-side shifting has carried the Mexican pebbles to where they are now. The story (summed up in figure 1.4) goes like this: About forty million years ago, a now extinct river, known to geologists as the Ballena River, flowed southwest from the Mexican volcanoes, carrying the distinctive pebbles toward the ocean. (The Gulf of California had not yet opened, so the river flowed uninterrupted to the Pacific.) About eighteen million years ago, sidling movements between the Pacific and North American plates began to split the pebbly deposits of the old riverbed. The easternmost section of the riverbed remained near its source in Sonora. The middle section slid northwest to where San Diego is now. The western section — the part of the river that poured into the ocean to form a deep-water delta — slid farther northwest, to where San Miguel and the other Northern Channel Islands are today (figure 1.4).
This story links directly to Southern California's large-scale geologic evolution, the details of which I'll give you in chapter 4. But you may be wondering: What's the connection here with deep time? Remember that the Pacific Plate slides past the North American Plate at just two inches per year — a spectacularly slow rate in human time. (A snail that fast would take three and a half centuries to cross my sixty-foot-wide suburban backyard.) But watch what happens when deep time comes into the picture. Two inches per year times eighteen million years (about how long ago the side-by-side movements between the two plates began in Southern California) equals 36 million inches, which is 540 miles — or almost exactly the distance that the pebbles on San Miguel Island now lie from their source volcanoes in Mexico. The message is as simple as it is powerful: Processes that seem trivially slow over human time can accomplish stunning work over geologic time.
THE PACIFIC PLATE–NORTH AMERICAN PLATE BOUNDARY
The story of the migrant pebbles on San Miguel Island highlights the most important geologic force at work in Southern California: side-by-side sliding of large blocks of the Earth's crust along big faults. Figure 1.3 shows that most of these faults trend northwest–southeast. This makes sense; that alignment allows the Pacific Plate to slide northwest past the North American Plate. Earthquakes happen whenever movements between the two plates cause one of these faults, every now and then, to snap. The San Andreas fault is the longest and most important — it's the big gorilla of California's faults — but the San Andreas doesn't act alone. Dozens of faults, spread across a zone more than two hundred miles wide, allow the side-by-side movement of the two plates to happen.
This idea — that the boundary between the Pacific and North American plates is a wide zone of shifting faults rather than a single fault — is vital for understanding the geology not just of Southern California, but of the entire western United States. To see what I mean, look at figure 1.5, which shows the results of precision GPS measurements made at various places across the western United States over the past several decades. The arrows show how fast various places are moving in relation to the continental interior. One way to visualize this is to imagine pounding a huge nail through, say, Kansas to pin North America in place; those arrows in figure 1.5 show how various areas west of the Rockies would still move. (In other words, western North America is slowly tearing apart — a topic to which I'll return in a moment.) You can see that the fastest movements (longest arrows) lie on the Pacific Plate, showing that it moves northwest about two inches per year in relation to the continental interior. Slightly to the east, notice the cluster of arrows on the area marked as the Sierran Plate, showing that it moves northwest about one-half inch per year. The Sierran Plate,* which includes California's Great Central Valley and Sierra Nevada, seems to be dislodging from the rest of the continent because of what we call the Big Bend in the San Andreas fault (marked in figure 1.3). Because of the curve in the fault at the Big Bend, the Pacific Plate pushes like a shrugging shoulder against the south end of the Sierran Plate, shoving it north and cracking it away from the land to the east. But as the arrows in figure 1.5 show, the Pacific Plate is moving northwest faster than the Sierran Plate, and that means the rocks along the Big Bend are being severely crushed. That gives the Big Bend region of the San Andreas fault its other geologic moniker: the Big Squeeze. The crushing forces within the Big Squeeze make it one of California's most earthquake-prone regions — a story that we'll explore in chapter 3.
Besides earthquakes, another product of the Big Bend/Big Squeeze is the Transverse Ranges, shown in detail on the map of Southern California at the front of the book. The Transverse Ranges — which include the Santa Monica, Santa Ynez, San Gabriel, and San Bernardino mountains — get their name from their east–west alignment, transverse to the mostly northwest–southeast alignment of other mountains in California. Pinch a watermelon seed between your thumb and forefinger. Your thumb represents the Pacific Plate and your forefinger the Sierran Plate, pushing against each other in the Big Squeeze. The seed is the Transverse Ranges, popping upward to escape the pressure. Some parts of the Transverse Ranges are rising nearly one-half inch per year, making them the fastest-growing mountains in North America. They are also — not coincidentally — some of the steepest, most landslide-prone, and most earthquake-prone mountains in the nation.
Returning to figure 1.5 and continuing east, notice the arrows across the Basin and Range Province, which spreads across all of Nevada and parts of neighboring states. The lengths of the arrows decrease eastward, telling us that points on the west side of the Basin and Range are moving northwest faster than points on the east side. In other words, the entire Basin and Range is stretching — the opposite of what is happening in the Big Squeeze. That stretching, which began some fifteen to twenty million years ago, explains why the Basin and Range looks the way that it does. The region gets its name from its washboard tempo of mountain ranges and intervening basins (valleys), all lined up generally north–south and divided from one another by large faults. Stretch the Earth's crust east–west, and it will break apart along north–south aligned faults. Blocks of rock that are rising along these faults make the ranges of the Basin and Range, while blocks that are dropping make the basins (valleys) in between. Since its inception, the Basin and Range has stretched by more than two hundred miles in some areas. And the work goes on. Each year, the drive between Reno and Salt Lake City increases by about half an inch (GPS measurements prove it). The American West is a living landscape, reshaping itself a bit every year as the Pacific Plate grinds northwest past the North American Plate and drags pieces of our continent along with it.
Before we return to Southern California, let me speculate a bit on what those movements shown in figure 1.5 might mean for the future of the western United States. In figure 1.6, I propose three not-too-fanciful scenarios for what the western part of the country may look like in the geologic future. Each scenario portrays a possible geography about fifteen million years from now. Each assumes that the Pacific Plate will continue to scrape northwest past the North American Plate at its present rate of two inches per year.
In scenario 1 in figure 1.6, the San Andreas fault takes over as the main locus of side-by-side motion between the two plates. Baja California and coastal California, including the Los Angeles and San Diego areas, shear away from the rest of the continent to form a long, skinny island. A short ferry ride across the San Andreas Strait connects Los Angeles to San Francisco.
Excerpted from Surf, Sand, and Stone by Keith Heyer Meldahl. Copyright © 2015 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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